Molten metal has long been formed into useful shapes both by batch processing techniques in which the melt is poured into discrete molds and by continuous casting techniques.
Metal sheet or strip materials are conventionally prepared by casting a block of base metal in a mold and subjecting the block to progressively thinner rolling until it is as thin as desired. This is an expensive and extensive process requiring major capital investment in expensive machinery and further requiring considerable processing effort and energy.
Some types of continuous casting processes simulate batch casting by forming a continuous series of molds which travel past a source of melt and are continuously fed and filled with melt. As the filled molds progress along a line of travel, the metal cools and solidifies in the conventional manner. The cast objects are thereafter removed from the molds. Such a system is illustrated by U.S. Pat. No. 3,587,717.
A similar continuous casting process is shown in U.S. Pat. No. 4,212,343. An elongated strip is formed by continuously pouring the melt against a mold surface which has surface contours or shapes which are replicated in the surface of the sheet to provide special imprints or other surface features.
Continuous casting by means of direct casting technology has been used commercially to form various products. In direct casting, the molten metal is applied against a moving chill block surface upon which it is solidified. It is then stripped from the surface. A variety of direct casting techniques have been disclosed in the prior art including melt spin or jet casting, melt extraction, planar flow casting, melt drag and pendant drop casting. More recently melt overflow casting has been explored.
In order to form the commercially successful wire products of the prior art by direct casting, a disk, or alternatively a cylinder having circular or helical ridges simulating a plurality of side by side disks, is brought into contact with the melt at its outer periphery. The melt solidifies on the tips of the peripheral ridges and is then stripped away to form wire. Techniques of this type are illustrated in U.S. Pat. Nos. 3,838,185 and 3,871,439.
The wire making concepts of direct casting have been extended to produce flakes of metal by forming the surface of a rotating chill block into a series of islands or "lands" which extend outwardly from the rotating chill block surface. In making flakes, only the top surfaces of these islands are inserted into the melt. The melt chills and solidifies only upon these islands in order to form the discontinuous, discrete flakes. This technique is represented by U.S. Pat. No. 4,154,284.
The prior art has further suggested that elongated ribbons or strips of sheet material may be formed by applying a molten material to the exterior, smooth surface of a slowly rotating roll. Systems for accomplishing this are illustrated in U.S. Pat. Nos. 105,112; 905,758; and 993,904.
The prior art attempts to form ribbon-like, sheet material using direct casting have met with some difficulty. First, the strip product which has been formed has been too thin for significant commercial use and its thickness has been too difficult to control. This is because the melt which does solidify on the rotating roll only solidifies in a very thin layer on the order of two to five thousandths of an inch thick. There is a need for a system which permits reliably accurate control of the product thickness and permits production of a considerably thicker product with the economies of direct casting. A thicker product can be passed through a simple rolling operation to provide metal strip of a commercially acceptable uniformity and thickness.
Another problem with sheet materials formed in the past by direct casting techniques is that the sheet products have both a nonuniform thickness as well as nonuniform physical and chemical properties along and across the strip. I theorize that this occurs because the solidfying melt does not contact the rotating surface of the chilling substrate in a uniform manner. Instead, I believe that relatively large air pockets collect and form at random regions between the solidifying melt and the surface of the rotating, chill block substrate. The metal at these regions is in contact with the roll surface and therefore the rate of heat transfer to the roll is relatively smaller in those regions relative to the rate of heat transfer at other regions where there is good contact. The result of the difference in heat transfer rate is not only thinner regions but also regions of different physical properties and even different chemical composition. These regions are distributed in an uneven, nonuniform manner along the strip.
Yet another problem which arises from these uneven, large areas of noncontact between the metal and the chill surface is that these large, noncontacting regions will not be quenched sufficiently fast. Because of the speed at which the solidifying layer travels through the process, the strip will be removed while the solidifying metal is still at a temperature which is so high that the metal in these regions is still brittle. The result is that the strip will exhibit breaks, cracks, porosity and other defects.
In summary, the resulting products of the prior art tend to be insufficiently thick, their thickness is difficult to control and they exhibit a nonuniform thickness and a nonuniform distribution of physical and chemical properties.